Because of the fundamental structure of the disc brake, which restrains and halts the rotation of the disc rotor by urging braking pads on a rotating disc rotor, a great deal of heat is inevitably produced between the braking pads and the disc rotor. And this frictional heat is gradually accumulated there to raise the temperature of the brake, which tends to shorten the life of the braking pads. Since the coefficient of friction of the braking pads (particularly in case of braking pads including organic materials) degrades, as is well known, in proportion to the rising temperature, it is important to prevent or diminish the temperature rising around the disc rotor and braking pads to stabilize the function of the disc brake.
Various propositions have been made for cooling the brake. A ventilated disc rotor wherein a number of air passages are formed within the disc rotor is one. The air passages in this case are formed in a space sandwiched by a pair of friction surfaces, on which braking pads are urged, from the central part thereof towards the outer periphery. In this type of disc rotor, the air in the air passages moves toward the outer periphery of the friction surfaces and expelled by the centrigual force produced from the rotation of the rotor, just like the impeller of a centrifugal fan. The disc rotor is cooled, in this instance, by the air flow passing through the air passages disposed in the friction surfaces.
This type of ventilated disc rotor is at present widely used and recognized for its effective cooling capability. However, it is not completely satisfactory. More efficient cooling capability of the disc brake is still required and wanted in many quarters. In fact, overheating of the brake is still liable to happen when the brake is frequently applied, because the car is not being driven very fast, thus the rotational speed of the disc rotor is slow, resulting in decelerating the air flow in the disc rotor, and degrading the cooling efficiency of the rotor. Further, to remodel the dust cover so that it is open in the car-driven direction, in pursuit of high cooling efficiency, brings a large volume of air into the brake system at the cost of inhaling undesirable water, mud, etc., mixed within the air. It spoils the inherent function of the dust cover.